But a deal's a deal, and sometimes that means shopping at Men's Wearhouse.

The attorneys at Willkie Farr & Gallagher, which advised Men's Wearhouse on its recent deal to purchase rival Jos. A. Bank, shrugged off Armani and Zegna in favor of $600 suits by Joseph Abboud —a once-prestigious brand now owned by Men's Wearhouse.

They were "custom-made," points out partner Steven Seidman, who says he donned a light blue number with a subtle plaid pattern to several meetings during the six-month negotiation period.

"If you're coming in to make a pitch to a client that you're going to be charging a lot of money to, you should understand their product," says David Edwab, vice-chairman of the board of directors for Men's Wearhouse.

In the hypercompetitive world of mergers, acquisitions and initial public offerings, you really do need to dress to impress. And it helps to have a flexible definition of what that means.

Consider the IPO of yogawear maker Lululemon Athletica seven years ago. Anxious bankers were prepared to strike a brand-appropriate pose to win the company's business.

Ditching their pressed shirts, suits and dress shoes, deal teams at several banks showed up at meetings wearing form-fitting yoga pants, track suit tops and sneakers to convince Lululemon's management that they would be committed to the underwriting assignment—and to the spirit of the brand.

But yoga isn't for everyone. The stretchy bottoms were tight, remembers one banker who pitched the company. "It was pretty embarrassing, actually," says the banker, who remembers leaving his hotel the morning of the pitch and feeling goofy as people in business suits walked by. Even though they were determined to wear the pants, the bank didn't get a piece of the deal.

In uptown Manhattan, UBS's bankers were planning an elaborate stunt of their own for the Lululemon pitch. The bank outfitted around 75 of its employees in Lululemon gear and had them descend upon Central Park for a "flash mob" yoga session. They photographed the event and presented it to Lululemon's management during their pitch. UBS was named as one of the deal's underwriters.

"Our sales went down when we stopped interviewing investment banks," jokes Robert Meers, Lululemon's CEO at the time, who was stunned by the number of bankers who loped into meetings sporting his product.

While bankers may resort to gimmickry for all manner of pitches, deal makers agree that retail clients require particularly special handling.

To woo a food company, for instance, bankers might arrange to have nibbles from the brand's pantry at the pitch. Tech bankers often dress down to be in sync with the relaxed vibe of Silicon Valley.

But fashion CEOs tend to have a discerning eye for detail and an obsession with who wears what.

True Religion founder and former CEO Jeffrey Lubell remembers one banker's entire outfit from when the two met in 2005.

"At the time, Peter Comisar appeared in my newest Big T Cords, a great shirt and blazer," recalls Lubell. "His fashion was a stand out in a sea of dark suits and ties."

Even the banker can recite his outfit for that crucial day: The corduroy pants were olive green with white stitching around the pockets.

The deal didn't materialise. But Comisar made such an impression that, years later, Lubell hired him to represent the brand in its sale to a private-equity firm.

The tradition of playing dress-up for a pitch extends back decades. Gilbert Harrison, founder of retail investment bank Financo, remembers learning the lesson the hard way. In the 1970s, Harrison showed up to a meeting in St. Louis with the chairman of Interco Inc. wearing his go-to shoe: black Gucci loafers. Interco owned a number of shoe brands at the time, including Florsheim.

"Young man if you're coming here to sell a Camel, don't smoke a Lucky Strike," Harrison recalls being told by Maurice Chambers, an Interco executive who is now deceased. Later on, Chambers gave him a catalog and told him to order a few pairs. Harrison chose dress shoes in black and brown, and never got off on the wrong foot with Interco again, he says.

Fashion faux pas can be costly. Karen Goodman, a managing director at Financo, recalls driving to a pitch to sell a business for a different shoe retailer. Goodman says she usually wears the products of the client she is pitching. But on her drive over to the meeting, she realized she had on a competitor's kicks.

"It was 8 a.m. and stores weren't open," she laments. Financo lost the deal to a competitor and later learned, from a board member, that the snub had to do with the not-so-fancy footwork.

All the wardrobe changes can add up. Bankers are typically expected to pay for outfits and accessories out of pocket, which can get pricey when it comes to pitching luxury clients.

Jane DeFlorio, a former retail investment banker at Deutsche Bank, still grits her teeth when she opens her jewelry box and finds two cheap-looking necklaces she bought for a meeting with the founder of an accessories brand. The opera-length cream and gold plastic disk necklaces set her back $500 each, plus expedited shipping, she recalls.

"It just killed me to buy them," she says. To make matters worse, spending big bucks didn't work. The company chose a different bank.

Sometimes, the sartorial solicitation makes for delicate business. When DeFlorio worked on bra and panty maker Maidenform's initial public offering, she was often the only woman in the room, she says. Whenever the management team or other bankers would talk about the fit or comfort of the bras, they'd motion to her and ask her opinion, she remembers. Intimately familiar with the product, she obliged.

During a roadshow presentation in New York, DeFlorio remembers addressing a room full of men. To add a bit of levity to the session, she introduced Maidenform's management team with a tacit product endorsement that elicited laughs.

"I can speak to the quality of this company, the quality of its management team," she said, jutting out her hips for the punch line. "And most of all, I can speak to the quality of the product."